Electrical stimulation of the inner ear has been very successful in restoring sound sensation to patients afflicted with deafness. Intra-cochlear electrodes are intended to restore some sense of hearing by direct electrical stimulation of the neural tissue in proximity of an electrode contact. The electrical stimulation is accomplished with an implanted cochlear implant stimulator connected to an electrode inserted deep into the scala tympani cavity.
But the insertion of the electrode causes a variable amount of connective tissue growth and trauma. The amount of trauma is very difficult to predict and depends on the cochlea anatomy, the electrode design, and the insertion technique. The trauma inflicted to the tissues may subsequently cause apoptosis and/or necrosis of nervous tissue (i.e., hair cells and spiral ganglion cells). Tissue growth and trauma may limit the performance of the implant. And trauma to spiral ganglion cells is cumulative and cannot be undone in the present state of technology. As more patients with significant usable residual hearing receive a cochlear implant, it becomes ever more important to use a minimally traumatic electrode. And as more patients are implanted at a young age who will be re-implanted several times during their lifetime, each consecutive insertion should limit the trauma to spiral ganglion cells to a minimum.
Trauma is usually caused by the electrode insertion into the delicate tissue of the inner ear. Insertion requires mechanical forces to be applied on the electrode to overcome the friction of the electrode against the tissue of the spiraling cochlea. To reduce trauma to the organ or tissue, electrodes and catheters should be soft and flexible, and insertion forces should be minimum. Unfortunately, most cochlear implant electrodes on the market today require significant force to be inserted, even for distances which are much less than the full length of the scala tympani.
The force required to insert an electrode or catheter is related to its size, geometry, and fabrication material. Material used in such devices includes materials for wires, contacts, functional metallic or polymer segment, and bulk material. The size of the device, the rigidity of the material used, the hydrophobicity of the outer shell of the electrode array, the energy stored in one way or another in the electrode, and the insertion process of the device all have an impact on the amount and location of tissue damage that will be inflicted during electrode placement.
Damage and trauma cause bleeding, inflammation, perforation of soft tissue, tears and holes in membranes, and fracture of thin osseous structures. The resulting damage may cause loss of surviving hair cells, retrograde degeneration of the dendrite which inervates the organ of Corti, and in the worst case, spiral ganglion cell death in the Rosenthal's canal. Cell death means that quantitatively less neural tissue is available for stimulation, and qualitatively that fewer frequency-tuned fibers are available to represent frequency information. Further loss of hair cells and loss of dendrites without loss of spiral ganglion cells means that acoustic stimulation is no longer possible, and that no synergetic effects between acoustic and electric stimulation is available. Electro-acoustic synergetic effects may be important for good sound discrimination in noisy environments.
Another inconvenience with cochlear implants is the rise in measured electrode impedance post-surgery. This rise is thought to be caused by encapsulation of the electrode by a tight membrane which reduces the efficiency of electric stimulation by creating a zone with ionic depletion around the contacts. It would make sense to post-surgically introduce some medicine into the cochlea to maintain a lower electrode impedance. It has been demonstrated, for example, that the introduction of cortico steroids can reduce the impedance rise after surgery. This has been done by depositing or rubbing the medicine on the electrode. But as the electrode is introduced in the fluid of the scala tympani, the medical solution quickly dissolves and may not reach a location where it would be most beneficial.
There have been attempts with non-cochlear implant patients to deliver medicine to the inner ear for the treatment of Meniere's disease or vertigo. The drug delivery takes place through the somewhat permeable round window membrane after injection of a bolus into the middle ear. One problem with round window drug delivery is that the membrane permeability to molecular substances changes over the course of a day, and that large molecules cannot pass through the tight membrane. It is thought that very little pharmacologic substance reaches the cochlear region beyond the first few millimeters of cochlea length.
There is no easy existing way to deliver medicine into the inner ear after cochlear implantation. The middle ear is not easily accessed and the inner ear is a sealed system that does not allow direct deposition or injection of medicines except at the time of cochlear implant surgery. After surgery the cochlea is partially filled with the electrode which should not be moved or removed.
Drug eluting electrode leads with cortico steroids have been used successfully in the past with cardiac pacemaker electrodes to reduce the contact impedance. In addition, silicone elastomer loaded with a pharmacological agent has been used as an eluting structure in several applications such as birth control, vascular injury treatment, and stents. Drug eluting electrodes have not been used with cochlear implants.